Electrical power cables enclosed in a protective outer sheath or tube are well known. The protective sheath is generally intended to withstand impact damage and, just as importantly, corrosive damage generated by moisture- or corrosive chemical-laden environments in which the cable is installed. Depending on its particular construction and application, the cable sheath may be designed to provide a liquid- and gas-tight enclosure for use in such hazardous environments. Accordingly, it is important that the sheath remain intact under all anticipated service conditions to maintain structural integrity of the cable and prevent detrimental intrusion of moisture or corrosive chemicals into the cable.
Generally, continuously-welded sheathed power cables are comprised of an assembly of insulated conductors enclosed in a flexible outer sheath. A filler material is provided between the conductors and the sheath to stabilize and isolate the conductors one from another and to provide some measure of electrical insulation to each conductor. An important problem, however, is that a breach of the outer sheath of the prior art undesirably enables entry and migration of moisture and other corrosive agents into the interior of the cable and along the length of each conductor, detrimentally affecting cable performance and longevity.
Various approaches have been taken to provide a strong, durable, and leak-tight sheath for power cables. For example, the sheath of a power cable disclosed in U.S. Pat. No. 3,766,645 is closed by forming the longitudinal edges of the sheath into a radially outwardly extending tab-like seam along the length of the cable. After the tab is trimmed to a uniform radial height, the cut edges of the tab are joined by a weld bead to seal the enclosure and to maintain electrical continuity of the sheath. The tab is then bent over to circumferentially abut the outer periphery of the cable.
An important problem with this approach is that discontinuities in the single weld bead may allow infiltration of moisture and other undesirable corrosive agents and gases into the cable.
According to a second approach, disclosed in U.S. Pat. No. Re. 30,228, an overlapping seam of a corrugated tape extends in a longitudinal direction of the cable. According to this approach, the longitudinal edges of the tape overlap one another without a crimp or fold. Fluid and gas integrity of this cable is provided solely by an outer coating of polyethylene or the like, rather than by the overlapping tape edges. Thus, the seam of this patent does not, by itself, provide a moisture- and gas-tight enclosure.
A third approach, disclosed in U.S. Pat. No. 3,662,090, includes a longitudinal tape seam which is folded over with a single fold, the seam then being compressed to a thickness less than that of the remainder of the tape. According to this patent, the fold must provide permanent contact between the overlying tape folds to maintain electrical continuity across the seam. Any non-conductive coating provided between the overlying tape folds could adversely affect the required electrical continuity of the assembled cable.
Yet another approach disclosed in U.S. Pat. No. 4,830,689 comprises a metal sheath having pre-bent, longitudinally-extending edges which cooperatively overlap to form, in cross-section, a trapezoidal joint. An adhesive material is provided within that joint to adhere the sheath edges together, while forming a leak-tight sheath closure. A problem with this approach, however, is that the strength and fluid integrity of the sheath closure is derived solely from the strength, volume, and distribution of the adhesive material because there is no substantial mechanical connection made between the overlapping sheath edges.
A further approach, disclosed in U.S. Pat. No. 3,073,889, provides a metal tape configured into a protective sheath about an electrical cable. The lateral edges of the tape are formed into a box seam along the longitudinal extent of the cable, followed by a coating of a thermoplastic material applied to the entire periphery of the cable assembly to provide an outer hermetic seal thereto. A thermoplastic filament is provided at the radially inner side of the box seam, and when heated, flows only to the immediate vicinity of the meeting edges of that portion of the box seam and between the conductors adjacent to the box seam.